(CNN) -- Violence like this week's shooting deaths of eight U.S. service members and an American contractor by an Afghan military pilot in Kabul stems from "isolated" incidents and is not part of any organized pattern of Afghan troops killing fellow service members in NATO's International Security Assistance Force, U.S. Army Lt. Gen. William Caldwell said Friday.

The growing frequency of so-called "green-on-blue" violence -- Afghan forces attacking NATO troops -- has deeply concerned NATO officials. In the past two years, 36 NATO deaths have been attributed to persons perceived to be Afghan soldiers or police.

A NATO analysis of 16 cases between March 2009 and this month "suggests that green-on-blue incidents are isolated and there are no clear patterns," Caldwell, the American general in charge of training Afghan troops, told CNN. A total of 25 Americans were killed in those 16 cases, according to a summary chart provided by Caldwell.

The analysis didn't include this week's nine American deaths in Kabul, and Caldwell's comments didn't address the incident, which the NATO-led force is investigating.

"The Afghan screening process creates sufficient barriers to infiltration attempts" by the Taliban, Caldwell said in written remarks to CNN. "Impersonation rather than infiltration is a greater concern."

As with past incidents of violence by Afghan forces against NATO personnel, the Taliban took credit for this week's gunfight by Afghan military pilot Ahmad Gull against the Americans at North Kabul International Airport.

But NATO officials disputed the Taliban's claim, and the pilot's brother told a local Afghan TV station that Gull wasn't part of the Taliban and had been suffering mental illness from past plane crashes and financial problems.

"However, both Afghans and ISAF forces understand the dangers of the insider threat and are vigilant. Given the importance of safeguarding Afghan and Coalition forces from the insider threat, (the Afghan National Security Forces) takes the threat of infiltration seriously," Caldwell said.

"We have no evidence that insurgents successfully embedded individuals to join the army or police with the intent to attack coalition forces," Caldwell added.

An eight-step screening process denies insurgents the anonymity they need to infiltrate the Afghan military and police, and the process uses technology and an examination relying on cultural practices, Caldwell said.

"The insider threat is real. The expansion of the Afghan Army requires that more emphasis be placed on the screening and vetting of new personnel as well as the spotting and assessing of those already in the ranks," Caldwell said in his written remarks.

Of 16 cases of violence by Afghan forces against NATO counterparts, seven can be attributed to combat stress or personal disagreements, five to soldiers with sympathies for insurgents, and four are undetermined, Caldwell said.